The W. K Mammel, U.S. Pat. No. 3,466,079, granted Sept. 9, 1969, assigned to Western Electric Company, Inc., describes a device employing the Bernoulli principle for picking up semiconductor wafers. The patent points out that alternative devices such as tweezers, vacuum pick-up devices, and other clamping and gripping devices may result in damage or contamination of semiconductor wafers from which highly complex integrated circuit chips are to be made. The Mammel device comprises a head member having a flat major surface through which an air passage extends. The head member is bought into proximity with the wafer while a significant air pressure is directed through the passage at the wafer. As a consequence, the air velocity on one surface of the wafer greatly exceeds that on the other surface to create a pressure differential in accordance with the Bernoulli principle. When this pressure differential is sufficient to overcome the force of gravity, the Mammel device may be used to pick up the wafer without contacting it to any significant degree; rather, a cushion of flowing air separates the wafer from the head member of the pick-up device. A plurality of peripheral extensions on the head member may contact the periphery of the wafer so as to stabilize it.
Wafer pick-up through the use of the Bernoulli principle requires a stable flow of air for giving a stable pressure differential on opposite sides of the wafer. Instabilities can cause the wafer to tilt which in turn changes the air flow distribution and causes the wafer to drop; sometimes the wafer vibrates and may break by repeatedly striking the head member. Greater stability can be achieved in the Mammel tool by providing a large plurality of air passages symmetrically disposed around the center of the head member, rather than a single passage through the center of the member. This, of course, complicates construction of the device, and care must be taken to assure a symmetrical distribution of the pressurized air directed toward the wafer. Moreover, as the diameters of the semiconductor wafers to be picked up with devices of this type increase, the degree of instability likewise increases. Current silicon wafers have diameters of five inches or more, and the expectation is that requirements for wafer diameter will increase in the future rather than decrease.
Related to the problem of instability is the problem of using the Mammel device to vertically orient a wafer. The use of epitaxial reactors requires, at one stage in the processing of a wafer, that the wafer that is initially in a horizontal plane be picked up by a pick-up tool, rotated ninety degrees so as to lie in a vertical plane and then moved horizontally so as to be mounted on a vertical wall within the epitaxial reactor. A Bernoulli principle pick-up tool that can move a wafer from one location to another while maintaining it in a horizontal plane may not work for moving it to a vertical plane because small instabilities may cause the wafer to drop. Gripping elements are sometimes used to overcome this problem, but as was mentioned before, it is desired that the dependence on the gripping of semiconductors be minimized.
For the foregoing reasons, there is a need in the semiconductor industry for a tool for picking up and manipulating a semiconductor wafer, including orientation of the wafer in a vertical plane, while keeping physical contact between the tool and the wafer to a minimum.